1. Field of the Invention
This invention relates to a data recording medium such as a write once type optical disk, a method of recording data onto the recording medium, and an apparatus for and method of reproducing the recorded data therefrom.
2. Description of the Prior Art
As a write once type optical disk, WORM (Write Once Read Many) type optical disk is known. In the case of recording data onto such a write once type optical disk and reproducing the recorded data therefrom, accurate tracking control must be carried out similarly to the case of other types of optical disks.
As a tracking servo system of a write once type optical disk, there are a Continuous Servo system (hereinafter referred to as "CS system") known as a continuous groove system, and a Sampled Servo system (hereinafter referred to as "SS system").
A recording/reproducing apparatus for a write once type optical disk is shown in FIG. 1. As shown in FIG. 1, the write once type optical disk 1 is rotated by a spindle motor 2. At the time of recording data, recording data D.sub.REC is optically recorded onto the optical disk 1 by means of a recording circuit 4 at timings of a write clock signal CLK.sub.W supplied from a write clock generator 3. Thus, recording pits each corresponding to the recording data D.sub.REC are formed on the optical disk 1. At the time of reproducing the recorded data, a reproducing signal X.sub.1 is reproduced from the recording surface of the optical disk 1 by means of an optical pickup (not shown) while the optical disk 1 being rotated. A data reproducing circuit 5 executes certain processing such as amplification or waveform shaping onto the reproducing signal X.sub.1, and the signal thus obtained is fed to an A/D converter 6 as reproduced data X.sub.2. An A/D converter 6 converts the reproduced data X.sub.2 into a digital data D.sub.OUT at timings of a read clock signal CLK.sub.R generated by a reproducing clock generator (not shown). The digital data D.sub.OUT is fed to a controller (MPU) (not shown).
In the above-mentioned recording/reproducing apparatus, in order to correctly reproduce the recorded data, it is necessary that the read clock signal CLK.sub.R has a correct phase relationship with respect to the reproducing signal. This is particularly important when the recording density of the write once type optical disk is increased and the transfer rate of data becomes high. Namely, if the data transfer rate is increased, a frequency of the read clock signal CLK.sub.R becomes high, and accordingly the window width for data detection becomes narrow. As a result, when any phase shift takes place between the reproduced data X.sub.2 recorded at the timings of the write clock signal CLK.sub.W and the read clock signal CLK.sub.R for reading out the reproduced data X.sub.2, the probability of erroneous data reproduction becomes high and the phase margin becomes small.
In this regard, in an apparatus for reproducing an optical disk according to a format of the CS system, the read clock signal CLK.sub.R is generated from reproduced data, so that a phase shift of the read clock signal CLK.sub.R becomes relatively small. However, when the recording density is increased, a phase shift may inevitably take place because of unevenness (variations) of the recording characteristics of respective optical disks, or unevenness (variations) of the delay characteristic of the reproducing system, etc.
On the other hand, in an apparatus for reproducing an optical disk according to a format of the SS system, the read clock signal CLK.sub.R is generated from prepits formed in advance on the optical disk. However, even in this case, phase shift may take place between the reproduced data X.sub.2 and the read clock signal CLK.sub.R because of unevenness (variations) of the recording characteristics of respective optical disks, unevenness (variations) of respective delay characteristics of the prepit reproducing system and the data reproducing system.
For correcting such a phase shift, one method predicts a phase shift which would take place in the reproduction at the time of recording and controls the timing for writing recording data D.sub.REC so as to correct the phase shift to be a minimum level at the time of reproduction.
However, according to such a technique for correcting the phase shift, any phase shift may take place by the influence of unevenness (variations) of the recording characteristics of optical disks or unevenness (variations) in the characteristics of respective reproducing apparatuses when disks having different recording characteristics are reproduced by the same reproducing apparatus or the same disk is reproduced by different reproducing apparatuses.